1. Field of the Invention
The present invention relates to a fixed airborne IFF antenna.
2. Discussion of Background
"Fixed" airborne antenna will be understood to mean a non-orientable antenna fixed onto the fuselage of the aircraft, generally under a flat radome, as opposed to IFF scanning antennas, to which category the antenna of the invention (which seeks moreover to remedy a certain number of disadvantages characteristic of these scanning antennas) does not belong.
The IFF technique (Identification Friend or Foe) operates on two levels: transponder ("surveillance" mode) and interrogator ("challenge" mode).
In the absence of information on the direction of the target to be identified, it is necessary to provide, in "surveillance" mode, omnidirectional coverage, this generally being achieved by means of two radiating elements whose pattern is, in azimuth, omnidirectional, each of the radiating elements ensuring, in elevation, a substantially hemispherical coverage. One of the radiating elements is placed on the back and the other on the belly of the craft.
By contrast, in the interrogation/response phase ("challenge" mode), directional antennas are used whose coverage corresponds approximately to that of the scanning of the radar on board the aircraft. This pattern, called the "fore directional pattern" or "sum pattern (.SIGMA.)" has been represented by .SIGMA. in FIG. 1 (it will be noted in this respect that "directional" means a pattern which, in contrast to an omnidirectional pattern, favours the fore sector, even if the aperture angle of this pattern is relatively wide, typically of the order of .+-.50.degree., a value corresponding substantially to the scanning sector of the on-board radar). The pattern V corresponds to the omnidirectional surveillance pattern.
The fore directional pattern is obtained with additional antennas, also two in number (back+belly), which are therefore added to the two omnidirectional surveillance antennas.
Moreover, there currently exist two different IFF standards, generally designated by the terms Eastern and Western, which use different frequency bands, respectively 600-700 MHz and 1030/1090 MHz.
Now, although it is known how to produce omnidirectional radiating elements which are bistandard, that is to say which operate equally well in both frequency bands, such is not the case for the radiating elements which make it possible to obtain the fore directional pattern.
Therefore, if it is desired that the aircraft be able to use one and the other standard, it will be necessary to provide a set of directional antennas for each standard, thus bringing to six the total number of antennas required to be able to operate equally well in accordance with both standards.
This is particularly penalising in the case of airborne antennas owing to the very great difficulty in positioning and installing the antennas at appropriate locations on the fuselage.
A first aim of the invention is to avoid this multiplication of antennas, by proposing a single universal antenna making it possible to selectively produce either an (Eastern/Western) omnidirectional pattern, or a Western directional patter, or an Eastern directional pattern.
It will thus be possible to reduce to two (back+belly) the total number of antennas which the craft will have to carry, instead of six.
Another aim of the present invention is to permit improved discrimination of the targets to be identified.
In fact, referring to FIG. 1, it can be seen that the fore directional pattern .SIGMA. has a certain number of secondary side lobes, so that a close-by target situated in the direction D2 of one of these secondary lobes, hence situated outside the field of the on-board radar and hence invisible to the pilot, will be able to produce, if it is close, a more intense signal than a distant target which might be situated in the direction D1 of the main lobe. In this illustrative case, the pilot will receive the response from the target situated in the direction D2, the target which he does not see, and not that from the target situated in the direction D1 and which will not be discriminated from the other targets.
This disadvantage is least with scanning systems, which use so-called "integrated" antennas placed on the same support as the antenna of the on-board radar and are hence subject to the same mechanical scanning as the latter, or alternatively which use the antenna of the on-board surveillance radar directly (the so-called "cross-band" technique), the interrogation then being carried out in a band (X band) other than the IFF surveillance.
Both these latter techniques are however used less and less, the first downgrading the performance of the radar and posing mechanical problems of bulkiness of the extra antennas added to the antenna of the radar, and the second not being able to be implemented by recent pulse-compression radars.
The only other existing discrimination technique is a technique relying on the use of an additional pattern, the so-called "monitoring pattern" or "difference pattern (.DELTA.)", which is an aft directional pattern such as that illustrated by .DELTA. in FIG. 1, and of a series of three successive coded pulses P1, P2 and P3 of equal amplitude.
To effect the discrimination, the IFF system will emit the pulses P1 and P3 (the so-called "interrogation" pulses) using the fore directional pattern .SIGMA. (interrogation pattern or sum pattern), and the pulse P2 (the so-called "monitoring" pulse) with the aft directional pattern .DELTA. (monitoring pattern or difference pattern).
Thus, in the abovementioned case of the two targets situated in the directions D1 and D2, as regards the target situated in the direction D1, pulses P1 and P3 of higher level than the pulse P2 (since the .SIGMA. pattern favours the fore direction and the .SIGMA. pattern penalises this same fore direction) will be received, whereas as regards the target situated in the direction D2, the opposite will be the case.
Thus, by comparing the level of the pulse P2, as perceived through the .DELTA. pattern, with that of the pulses P1 and P3, as perceived through the .SIGMA. pattern, it will be possible to neutralise the effect of the secondary lobes of the .SIGMA. pattern. Generally, only the signals coming from targets for which the level of P2 is at least 9 dB less than that of P1 or P3 are preserved, this corresponding to a given angle of discrimination about the direction D1.
However, such a discrimination technique requires that a third pattern, namely the aft directional pattern .DELTA. be available in addition to the omnidirectional pattern V and the fore directional pattern .SIGMA..
On an aircraft this would necessitate the setting up of two additional (back+belly) antennas, or even four additional antennas if it is desired to be able to apply this technique to the two standards, Western and Eastern, thus bringing the total number of antennas to eight or ten respectively.
Such a multiplication of antennas is incompatible with an aircraft, which means that this technique, although very effective, has hitherto been employed only in ground/air identification, and very rarely in air/air identification.
A second aim of the invention is thus to permit, without increasing the number of antennas, which will still be two (back+belly), the use of this technique on board an aircraft, without increasing the number of antennas and without having to resort to a scanning technique.